Do you really believe that the ONLY factor is people not getting married is the age gap and if we “closed” then everyone who be married?
What about all the petty factors people pass up on shidduchim for?
Here a good starting point the UJA does a study of Jewish life with detailed data on Yeshivish/Chasish households. If you want the truth go this and look for growth rate. Then we can see if the 3-4% NASI throws around is true
You can google it and look at the data